1. Field of the Invention
The present invention relates to a color cathode ray tube apparatus and, more particularly, to a color cathode ray tube apparatus in which the focus characteristics of an electron gun assembly for emitting three electron beams arranged in a line and passing through the same plane are improved.
2. Description of the Related Art
A color cathode ray tube apparatus generally has the following structure. That is, three electron beams emitted from an electron gun assembly arranged in the neck of an envelope are deflected by horizontal and vertical deflecting magnetic fields generated by a deflection device arranged outside the envelope, and a color image is displayed by horizontally and vertically scanning a phosphor screen. As such a color cathode ray tube apparatus, an in-line type color cathode ray tube apparatus using an electron gun assembly for emitting three electron beams arranged in a line and consisting of a center beam and a pair of side beams which pass through the same horizontal plane is used.
In general, the electron gun assembly of the color cathode ray tube apparatus has an electron beam forming section, which controls electron emission from cathodes, focuses the emitted electrons to form three electron beams and is constituted by the cathodes and a plurality of electrodes sequentially arranged adjacent to each other on the cathodes, and a main electron lens section constituted by a plurality of electrodes for focusing and converging the three electron beams obtained by the electron beam forming section on a phosphor screen.
In the above color cathode ray tube apparatus, in order to make the characteristics of an image drawn on the phosphor screen preferable, the three electron beams emitted from the electron gun assembly must be appropriately focused and converged in the entire area of the phosphor screen.
As a method of converging the three electron beams, for example, as described in U.S. Pat. No. 2,957,106, a method of inclining and emitting the three electron beams from the electron gun assembly is used. In addition, as described in U.S. Pat. No. 3,772,554, a method of converging the three electron beams such that, of the three electron beam through holes of the electrodes constituting the main electron lens section, a pair of side beam through holes are slightly externally decentered from the side beam through holes of the adjacent electrode on the electron beam forming section side is also used. Both the methods are popularly used.
However, even when the electron gun assembly is constituted as described above, in an actual color cathode ray tube apparatus, convergence errors of the three electron beams occur when the electron beams are deflected. For this reason, a color cathode ray tube apparatus having the following structure is used. That is, a pin-cushion-shaped horizontal deflecting magnetic field and a barrel-shaped vertical deflecting magnetic field are generated by the deflection device for deflecting the three electron beams arranged in a line and constituted by the center beam and the pair of side beams passing through the same plane, and the three electron beams arranged in a line are converged in the entire area of the phosphor screen by these ununiform deflecting magnetic fields. This color cathode ray tube apparatus is known as a self-convergence.multidot.in-line type color cathode ray tube apparatus, and this color cathode ray tube apparatus is prevalent at present.
However, when the three electron beams are converged by the deflecting magnetic fields generated by the deflection device, the three electron beams considerably receive the influence of deflection errors, and the distortion of a beam spot at the peripheral portion of the screen increases, thereby degrading a resolution. The degradation of the resolution caused by the deflection errors becomes conspicuous when a deflection angle increases from 90.degree. to 110.degree..
The degradation of the resolution at the peripheral portion of the screen occurs because, of three electron beams 1B, 1G, and 1R arranged in a line and shown in FIGS. 1 and 2, as shown in FIGS. 1 and 2 with respect to the side beam 1R of the pair of side beams, a focus operation is weakened in the horizontal direction (X-axis direction) but strengthened in the vertical direction (Y-axis direction) by a pin-cushion-shaped horizontal deflecting magnetic field 2H and a barrel-shaped vertical deflecting magnetic field 2V. As a result, as shown in FIG. 3, although a circular beam spot 3 is formed at the central portion of the screen, a beam spot 3 at the peripheral portion has a shape obtained by forming low-luminance halo portions 5 at the upper and lower portions of an oval high-luminance portion 4 having a horizontal major axis, and the resolution of the peripheral portion of the screen is considerably degraded.
A technique for reducing the distortion of the beam spot 3 at the peripheral portion of the screen caused by deflection errors to prevent degradation of a resolution is disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 60-7345 (U.S. Pat. No. 4,887,001), Jpn. Pat. Appln. KOKAI Publication No. 64-38947 (U.S. Pat. No. 4,897,575), or Jpn. Pat. Appln. KOKAI Publication No. 1-236554 (U.S. Pat. No. 5,034,652). In particular, in an electron gun assembly disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 60-7345 or Jpn. Pat. Appln. KOKAI Publication No. 1-236554, a beam spot at the central portion of a screen can be decreased in size. In a color cathode ray tube apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 64-38947, the distortion of a beam spot at the peripheral portion of the screen can be considerably decreased in size by a dynamic focus operation for changing the strength of the electron lenses of an electron gun assembly in accordance with a deflection amount, and an image having a high resolution can be obtained.
As described in these publications, this structure can be obtained such that an electron optical system for forming asymmetrical electron lenses in front of or behind the area of a normal symmetrical cylindrical electron lens is employed. However, in order to form such asymmetrical electron lenses, according to a conventional technique, a flange-like electric-field correction electrode is inserted into a bath-tub electrode, or electron beam through holes each having a horizontal major axis are formed.
As an example of this structure, an electron gun assembly in which an electric-field correction electrode is arranged is shown in FIG. 4. This electron gun assembly has three cathodes KB, KG, and KR arranged in a line, three heaters (not shown) for respectively heating the cathodes KB, KG, and KR, first to fourth grids G1 to G4 sequentially arranged adjacent to the cathodes KB, KG, and KR in the direction of a phosphor screen, and a convergence cup Cp arranged on the fourth grid G4. The cathodes KB, KG, and KR and the first to fourth grids G1 to G4 are assembled to have a structure integrally fixed by a pair of insulating support members (not shown).
In this electron gun assembly, each of the first and second grids G1 and G2 is constituted by a plate-like electrode in which three relatively small electron beam through holes arranged in a line in correspondence with the cathodes KB, KG, and KR are formed. The third grid G3 is constituted by a cylindrical electrode obtained by connecting two bath-tub electrodes G31 and G32 to each other, and the fourth grid G4 is constituted by connecting two bath-tub electrodes G41 and G42 to each other. Three electron beam through holes each having a diameter larger than each of the electron beam through holes of the second grid G2 and arranged in a line in correspondence with the cathodes KB, KG, and KR are formed in the surface of the third grid G3 opposing the second grid G2. Three electron beam through holes 8B, 8G, and 8R each having a diameter larger than each of the electron beam through holes of the surface of the third grid G3 opposing the second grid G2 and arranged in a line in correspondence with the cathodes KB, KG, and KR are formed in the surface of the third grid G3 opposing the fourth grid G4. Three electron beam through holes 9B, 9G, and 9R each having a diameter almost equal to that of each of the electron beam through holes 8B, 8G, and 8R and arranged in a line in correspondence with the cathodes KB, KG, and KR are formed in the surface of the fourth grid G4 opposing the third grid G3. Three electron beam through holes each having a diameter almost equal to that of each of the three electron beam through holes 9B, 9G, and 9R and arranged in a line in correspondence with the cathodes KB, KG, and KR are formed in each of the opposing surfaces of the fourth grid G4 and the convergence cup Cp. In addition, the pair of side beam through holes 9B and 9R in the surface of the fourth grid G4 opposing the third grid G3 are slightly externally decentered from the pair of side beam through holes 8B and 8R in the surface of the third grid G3 opposing the fourth grid G4 in the arrangement direction of these electron beam through holes. A pair of electric-field correction electrodes 10a and 10b are respectively arranged inside the opposing bath-tub electrodes G32 and G41 of the third and fourth grids G3 and G4 to vertically sandwich the three electron beam through holes 8B, 8G, 8R, 9B, 9G, and 9R.
In this electron gun assembly, a voltage obtained by adding a video signal voltage to a cutoff voltage of 200 V is applied to the cathodes KB, KG, and KR, the potential of the first grid G1 is set to be a ground potential, and a positive high voltage of 500 to 1,000 V, a positive high voltage of 5 to 10 kV, and a positive high voltage of 25 to 30 kV are applied to the second, third and fourth grids G2, G3, and G4, respectively. In this manner, high-performance electron lenses are formed between these electrodes.
Even when the electron gun assembly is constituted as described above, of the three electron beams arranged in a line and emitted from the electron gun assembly, the center beam can be preferably converged, but the pair of side beams are disturbed due to a coma of the electron lens. For this reason, a beam spot at the central portion of the screen is distorted. Moreover, when the beams at the peripheral portions of the screen are deflected, the beams receive more strong deflection errors, and a beam spot at each peripheral portion of the screen is considerably distorted.
Lens components acting on the pair of side beams of a main electron lens section formed between the third and fourth grids G3 and G4 are represented by vectors. For example, as indicated by arrows 11H and 11V in FIG. 5A, a quadrupole lens component for horizontally diverging and vertically focusing the side beam 1R acts on the side beam 1R on the third grid G3 side, and as indicated by arrows 12H1, 12H2, 12V1, and 12V2 in FIG. 5B, a prism component for deflecting the side beam 1R in the direction of the center beam acts between the third and fourth grids G3 and G4. In addition, as indicated by arrows 13H and 13V in FIG. 5C, a non-orthogonal quadrupole lens component for horizontally focusing and vertically diverging the side beam 1R in a direction inclined with respect to the vertical axis (Y-axis) acts on the side beam 1R on the fourth grid G4 side. As shown in FIG. 5D, the side beam 1R is influenced by the vector of a lens component obtained by synthesizing the above lens components except for the prism component. More specifically, as the operations of the synthesized lens component for the side beam 1R, focus vectors 14H having the same length act from both the horizontal sides to the center of the beam, and focus vectors 14V each having a horizontal component deviated from the center beam obliquely act from both the vertical sides. For this reason, the rotationally symmetrical side beam 1R free from distortion as shown in FIG. 6A is focused such that a vertical beam component has an arc-like shape as indicated by a broken line in FIG. 6B. This causes the electron beam to be distorted.
As a means for correcting the distortion of the electron beam, an electron gun assembly in which a correction plate having trapezoidal electron beam through holes is formed in an electrode constituting a main electron lens section is described in the Jpn. Pat. Appln. KOKAI Publication No. 4-267037. However, even when this correction plate is arranged in the electrode, only a weak correction operation is obtained. For this reason, when an electron lens having a non-orthogonal asymmetrical lens component is formed between opposing electrodes, a satisfactory correction effect cannot be obtained.
In addition, an electron gun assembly having the following structure is disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 5-3659. That is, opposing bath-tub electrodes are arranged, and an electrode in which three electron beam through holes are formed is arranged in each of the bath-tub electrodes, thereby correcting the multipolar lens components of an electron lens. In this electron gun assembly, a large-diameter electron lens commonly acting on three electron beams is formed by the opposing bath-tub electrodes, and this large-diameter electron lens becomes an electron lens having asymmetrical lens component having very strong orthogonality with respect to the pair of side beams. Therefore, in order to correct the asymmetrical lens component, each of the electron beam through holes of the electrode arranged in each bath-tub electrode has a polygonal shape. However, this electron gun assembly has a weak correction operation because the electrode is arranged in each bath-tub electrode. In addition, when the electrodes are arranged to be close to the opposing surfaces of the bath-tub electrodes to strengthen the correction operation, the effective diameter of the large-diameter electron lens decreases, i.e., a structural dilemma occurs. For this reason, a design for the electron gun assembly is limited.
In a picture tube, electron beams are not always focused in an optimal state on a phosphor screen due to variations in applied voltage or assembling of an electron gun assembly. For this reason, in general, a focus voltage is made variable, and the focus voltage is adjusted to obtain an optimal beam spot. However, in each of the above examples, a correction electrode is arranged between the opposing electrodes, and an electric-field permeated into the correction electrode is uniformed to correct the distortion of an electron beam. For this reason, when an optimal focus voltage is different from an optimal electron beam distortion correction voltage, a distortion correction operation for the electron beam becomes improper, and an optimal beam spot cannot be obtained.
As described above, in a self-convergence.multidot.in-line type color cathode ray tube apparatus which has an electron gun assembly for emitting three electron beams arranged in a line and constituted by a center beam and a pair of side beam passing through the same plane and which converges the three electron beams emitted from the electron gun assembly in the entire area of a phosphor screen by a deflecting magnetic field generated by a deflection device, the distortion of a beam spot at the peripheral portion of the screen increases due to deflection errors, thereby degrading a resolution. This degradation of the resolution becomes conspicuous when a deflection angle increases. In order to reduce the degradation of the resolution, electron lenses each having an asymmetrical electron lens component are advantageously formed in front or behind the lens area of a normal symmetrical cylindrical electron lens formed at the main electron lens section of the electron gun assembly. Therefore, an electron gun assembly in which degradation of the resolution is reduced by the above conventional method has been developed.
However, in the conventional electron gun assembly for reducing the degradation of the resolution, although the center beam of the three electron beams arranged in a line can be preferably focused, a non-orthogonal asymmetrical lens component acts on the pair of side beams, and the pair of side beams are distorted by a lens aberration. A beam spot is distorted at the central portion of the screen. In addition, when the beams at the peripheral portion of the screen are deflected, the beams receive more strong deflection errors, and a beam spot at the peripheral portion of the screen is considerably distorted, thereby degrading the resolution.
Although an electron gun assembly for correcting a non-orthogonal asymmetrical lens component with respect to a pair of side beams is conventionally developed, since this conventional electron gun assembly for correcting the non-orthogonal asymmetrical lens component locally uniforms part of an electric field permeated into electrodes for forming a main electron lens section, the conventional electron gun assembly does not have a sufficient sensitivity to correct the non-orthogonal asymmetrical lens component of an orthogonal asymmetrical electron lens system, thereby unsatisfactorily correcting the non-orthogonal asymmetrical lens component.